The judge issued an injunction against the American Quarter Horse Association and its prohibition of cloned horses, and their offspring, from its registry. The registry adds financial value to listed animals.

Two Texas ranchers had successfully sued the 280,000-member association, saying it was operating a monopoly by not allowing cloned horses. Jurors sided with the ranchers last month, ruling that association was violating antitrust laws, but the association — which has vowed to appeal — wasn’t immediately ordered to change its policy.

U.S. District Judge Mary Lou Robinson announced the injunction during court Monday. It will take effect 30 days after she signs it, which may happen later this week.

The decision could set a precedent because no American horse breeding groups currently allow cloned horses to be registered.

She said members have been asking that the rules be changed since late 2007, but she acknowledged that less than a half dozen members have actually requested the change.

The quarter horse association remained adamant Monday about its rights as a private group to set reasonable and lawful rules.

“AQHA will continue to take any and all necessary legal action in seeking to have the verdict of the jury and any judgment entered by the Court in favor of plaintiffs reversed,” AQHA Executive Vice President Don Treadway said in an emailed statement. “AQHA will continue to fight for its members’ rights.”

The ranchers’ attorneys noted during trial that the association has, since 1960, registered animals born through other non-natural means, including artificial insemination.

The cloning method at issue, called somatic cell nuclear transfer, is the most common means of cloning.

A somatic cell, which isn’t from a sperm cell or egg cell, is taken from the animal to be cloned and contains the complete DNA of the animal. An egg cell is then taken from a female of the same species. In the lab, a scientist extracts and discards the nucleus of the egg cell, which holds the egg donor’s genes and the somatic cell from the genetic donor is inserted into the egg. The resulting egg develops using the genetic donor’s DNA and is then implanted into a surrogate mother.

In 1997, scientists in Scotland announced the birth of Dolly, a sheep born a year earlier and the first animal cloned using the somatic cell nuclear transfer.